It is desirable, for economic and ecological reasons, for aircraft to have the lowest possible fuel consumption. Aircraft of the current generation are as a rule operated with kerosene, and it is assumed at present that kerosene-based fuels will also be the predominant fuel source in future. The costs for corresponding fuels are expected to rise further in future.
There are several approaches for lowering the fuel consumption of aircraft: For example, the use of new materials leads to weight reduction and therefore also to a decrease in fuel consumption. An important approach for lowering fuel consumption is to reduce the air resistance of aircraft. The total drag is greatly influenced by the flow behavior on the wing surfaces. Laminar and turbulent flows can occur on the wing. Laminar flows ensure much less drag compared with turbulent flows. There are two approaches for achieving laminar flow: on the one hand, it is possible to use wings that permit natural laminar flow. On the other hand, a laminar flow profile can also be achieved by means of so-called hybrid laminar flow control (HLFC):
In HLFC, a laminar flow profile is achieved by suction of the air flow through a porous surface. The suction stabilizes the boundary layer and makes laminar flow on the surface possible. This approach involves relatively high energy expenditure.
As already mentioned, it is sensible both ecologically and economically to reduce fuel consumption. In the case of aircraft, from the ecological standpoint decreased fuel consumption not only offers the advantage that there is less emission of greenhouse gases and saving of resources, but also that a decrease in fuel consumption has an especially beneficial effect, as aircraft produce their exhaust gas emissions at altitudes that are particularly sensitive to pollution.
Attainment of laminar flow profiles is already technically achievable. In practice, however, the laminar flow profile is often disturbed by contaminants. The contaminants are deposited on the surface and thus result in a rough surface. The roughness leads to turbulence, which disturbs the laminar flow. To ensure a laminar air flow in operating conditions, the resultant contaminants must be removed or at least greatly reduced. In the case of aircraft, the contaminants in question are mainly insects and possibly also icing. Contamination with insects occurs on the ground and in particular during take-off and landing, when insects collide with the aircraft and adhere to it.
Insects vary in size from 1 to 2 mm in the case of aphids and 20 mm in the case of moths. Insects can weigh up to 40 g. They consist essentially of two components, the exoskeleton and the hemolymph. The exoskeleton forms the protective shell and consists mainly of the polysaccharide chitin and the structural proteins sclerotin and resilin. The internal organs of insects are located in a free-flowing blood-like fluid, the hemolymph.
The hemolymph consists of proteins, which can coagulate and act like glue. As a result, parts of the exoskeleton adhere to surfaces [O'Donoghue et al., 2002].
For the occurrence of insects, mainly three factors are important:                vertical distribution        temperature and season        wind speed.        
It should be borne in mind that most insects are found at relatively low altitudes. In contrast, at the usual cruising altitude of scheduled planes there is little likelihood of encountering insects.
Croom and Holmes suggested that protection against insect contamination would only be necessary up to an altitude of 152.4 m (500 ft), because in the authors' opinion hardly any insects occur at higher altitudes. According to Coleman, however, altitudes up to 1524 m (5000 ft) should also be considered: during a typical flight of an aircraft, 54% of the insects have already been collected on the ground, 33% between take-off and an altitude of 304.8 m (1000 ft) and the remaining 13% at an altitude of 304.8 m to 1524 m. The authors Maresh and Bragg agree that most insects are collected on the ground and while climbing.
Insect activity depends on the temperature. Activity is greatest in a temperature range from 21° C. to 27° C. Elsenaar and Hasnoot reported that insect density is highest in summer and in spring.
At high wind speeds there is a sharp drop in insect density.
As well as insects, icing of the wings can also lead to a change of the aerodynamics. In contrast to insect contamination, icing is also a safety problem. Thus, icing of parts of the aircraft not only leads to termination or hampering of laminar flow, but the ice itself can also lead to a marked weight increase.
A number of approaches are already known in the prior art for protecting aircraft against insect contamination and/or icing in particular on the ground and against contamination, in particular by insects, during the take-off phase:
One basic approach is to cover the surfaces of aircraft requiring protection with a removable device. This can be provided e.g. by a layer of paper, which is removed after take-off or by the use of a deflector, e.g. a Krueger flap, which is retracted after passing through the insect zone.
Thus, DE 35 29 148 proposes providing a film to protect aircraft against insect strikes. This film is removed as a whole after take-off and drawn in by means of a retracting device.
Moreover, DE 39 46 403 A1 proposes a similar principle, wherein a protective cover is provided with an active solution device.
DE 20 59 492 describes a mechanically destructible protective cover.
This approach has the disadvantage that the protective devices permanently increase the weight of the aircraft, as the protective devices, for example the Krueger flap, must remain on the aircraft permanently or alternatively, in the case when they are discarded after fulfilling their function, they cause considerable disposal problems owing to their size. Moreover, application of the corresponding covers (especially when they are not integral with the aircraft) is as a rule very laborious.
An alternative approach is to use cleaning devices, which can also perform their function during flight. This category includes scrapers and wipers and deluge sprinklers, for washing-away insect contamination. Examples of such applications are disclosed e.g. in DE 40 20 585 and in DE 40 16 850.
These systems have the disadvantage that the cleaning action they provide is inadequate. Attainment of laminar flow can be disturbed by the presence of just one insect contamination with a height ≧40 μm per running meter, so these mechanical cleaning systems are inadequate for many applications. In addition, they also increase the weight of the aircraft to a considerable extent, and permanently.
A third approach known in the prior art is the application of a coating that remains permanently on the surface to be protected and should prevent or reduce contamination owing to the nature of their surface. One approach in this connection is the use of elastic coatings, which as it were “reflect” the contaminants, as proposed for example in GB 2 299 280 A. A corresponding approach is also used in DE 1 190 342.
These protective layers have the disadvantage that they increase the weight of the aircraft permanently and as a rule provide unsatisfactory protective action against contamination. It has to be borne in mind that, especially during take-off, insects are impacted with high force on the aircraft surfaces. Furthermore, these protective layers only offer inadequate protection against icing, in particular while waiting on the ground.
In earlier research, water-soluble films were described for preventing insect contamination (Coleman, W. S.: Wind Tunnel Experiments on the Prevention of Insect Contamination by means of Soluble Films and Liquids Over the Surface. Rep. To the Boundary Layer Control Committee, BLCC Note 39, 1952). In later research these approaches were rejected on the grounds of complexity and impracticability (e.g. application of water in possibly cold conditions) (Cynthia C. Croom and Bruce J. Holmes, “Insect Contamination Protection for Laminar Flow Surfaces”, NASA Langley Research Center Hampton Va.).